The present application claims priority to Japanese Application No. P11-177195 filed Jun. 23, 1999 which application is incorporated herein by reference to the extent permitted by law.
1. Field of the Invention
This invention relates to a novel optical recording medium for recording and/or reproducing information signals by exploiting changes in birefringence caused by changes in molecular orientation of an organic material. This invention also relates to a recording and/or reproducing method and an apparatus employing this optical recording medium.
2. Description of Related Art
Up to now, phase difference modulation by pits or crests/recesses of a dyestuff material, reflectance difference modulation between the amorphous and crystal phases, as in case of a phase change recording film, or detection of polarization by the photo-magnetic Kerr effect, has been known as a recording and/or reproducing principle for an optical disc.
The recording system in a recordable optical disc is thermal recording, the basic concept of which is that changes in the status of a recording film are caused at a certain threshold temperature to record the changes. The recording and/or reproducing signals in this case are non-linear and hence are suited for recording digital binary signals.
There is also known an optical recording system employing a photochromic material, known as a photon mode. This system has a number of drawbacks in storage stability following the recording, readout stability and in durability against repeated use, such that a solution which overcomes all of these problems has not been found to date.
Meanwhile, the recent tendency in the field of optical recording is towards a shorter wavelength of a light source and a higher numerical aperture (NA) of an objective lens. Thus, it has been reported that, by combining a blue purple light source and an objective lens with a numerical aperture NA=0.85, a recording capacity as large as 20 GB can be realized with an optical disc having a diameter of 12 cm. This, however, may be said to indicate that the recording and/or reproduction by a so-called far-field optical system is approaching to its theoretical limit.
Among the methods to overcome this limitation is a method of using a recording material capable of linear recording and/or reproduction to realize multi-values recording.
However, the thermal recording, which has so far been customary, is a non-linear recording system, such that it is difficult to realize multi-valued recording.
In this consideration, the recording material of the thermal recording system, such as a metal-based material used as a phase-change recording material or a photomagnetic recording material, may not be said to be desirable.
It is therefore an object of the present invention to provide a novel optical recording medium capable of achieving linear recording and/or reproduction and a recording and/or reproducing method and apparatus employing this novel optical recording medium.
For accomplishing the above object, the present inventors have conducted various researches for long, and have reached a conclusion that a novel optical recording system can be constructed by employing, as a recording material, an organic material which has its molecular orientation changed on light illumination to exhibit birefringence.
The present invention, completed on the basis of this finding, provides an optical recording medium having a recording layer including an organic material having its molecular orientation changed on illumination of polarized light.
The present invention also provides a recording and/or reproducing method including illuminating polarized light to a recording layer including an organic material as a recording material to change its molecular orientation to record information signals and reading out changes in birefringence caused by changes in molecular orientation as changes in the polarized state of the incident light.
The present invention also provides a recording and/or reproducing apparatus employing, as a recording medium, an optical recording medium including a recording layer containing an organic material having its molecular orientation changed on illumination of polarized light. The apparatus includes a recording optical system for illuminating pre-set polarized light as recording light on said recording layer, and a reproducing optical system for detecting the polarized state of the playback light incident on said recording layer.
As described above, the basic concept of the present invention is that such an organic material changed in molecular orientation on light illumination to exhibit birefringence is used as a recording material.
The recording system exploiting the changes in molecular orientation in the organic material exhibits linearity, such that different outputs are obtained depending on the degree or angle of orientation, thus enabling multi-valued recording.
According to the present invention, it becomes possible to construct an optical recording system which is totally different from the conventional system.
As meritorious effects of the present invention, the playback signals larger than those with a magneto-optical disc can be produced, while signals proportionate to the volume of illuminated light can be achieved, thus assuring linear recording and/or reproduction and high density recording by combination with e.g., multi-valued recording. The high quality transcription recording and erasure becomes possible with an organic material. Since the organic material can be prepared by coating by a spinning method, manufacturing cost can be reduced appreciably. Moreover, the signal level can be changed to meet system requirements by selecting the combination of the direction of orientation of the recording film and the direction of polarization relative to the track direction. In addition, readout and writing can be performed using the same wavelength by changing the direction of polarization.